The present invention relates to alternating current power measurement instruments and more particularly to electronic watt transducers and electronic var transducers.
Various types of solid state electronic wattmeters have been proposed and have been alleged to be of greater accuracy than electromechanical instruments, for example, the types shown in the following U.S. Pat. Nos. 3,875,508; 3,662,264; 3,900,794; 3,794,917.
The average power or watts "W" consumed in an element of an electric circuit with a flow of an alternating current, in which voltage E and current I are of the same frequency and are sinusiodal, is equal to EI cos .theta., where .theta. is the phase angle.
In a single-phase system the reactive power, sometimes called reactive volt amperes, or "vars", is equal to EI sin .theta..
It is known that an electronic watt transducer may be converted for alternative use as an electronic var transducer. For example, in U.S. Pat. No. 3,084,863 to W. B. Boast, a relay 110 is operated so that the instrument alternatively reads either watts or vars by switching in a resistor-capacitor network which shifts the incoming voltage by 90.degree.. In U.S. Pat. No. 3,723,872 to H. Rich, either three-phase watts or vars are alternatively measured, depending upon the position of a multiple-pole double-throw switch.
In these instruments the input to the transducer is the power from a power line. The transducer produces an electrical output which is proportional to the amount of electrical power consumed by a load or provided by a source. That output may be utilized in various ways. For example, it may be displayed by a meter or it may be accumulated over time to provide, as in a watt-hour meter, a measure of the amount of electrical energy consumed over time by a load, or provided over time by a source.